During transient processes, such as start, restart, shut-down or load changes in gas turbines and compressors, the distance between the rotor blades and the static components, such as heat-shields, are varying. Commonly, a rubbing of the rotor blades is circumvented by a sufficiently high distance between the rotor blades and the static components. This measure reduces the efficiency considerably.
As an alternative, oxidation- and wear-resistant abrasive coatings are applied at the rotor blade tips to prevent wear of the turbine blades upon contact with the static components, especially with the heat shields. These abrasive coatings contain abrasive particles, thereby enhancing wear resistance such that a sufficient protection of the turbine blades is provided, even if the static components are provided with a thermal barrier coating (TBC). Regularly, the abrasive particles consist of cubic boron nitride (cBN) as for example disclosed in DE 10 2010 049 399 A1. Because of its morphology and extremely high hardness, cBN has a superior cutting ability. However, at operating temperatures above 800° C. to 850° C. cBN is oxidizing with the result that a protective effect of an abrasive coating can only be provided for a very short time. cBN particles in abrasive coating can only guarantee a sufficient protection for about 1000 operation hours at temperatures above 900° C. Above 900° C. cBN particles are degraded rapidly by the oxidation process. Especially in the first turbine stages of a gas turbine, the temperatures can be between 900° C. to 1300° C.